Thanks for all the tips, but you all address the things I already said aren't the problem. This thing flies as if there's a deployed parachute at the front. Heavier bigger planes with lots of more drag get faster on the runway than this one.
Engines are at 100% ans the plane sill is slow. I had canards at the front. The fins were vertical once and I made many changes. I know that this isn't the easiest to fly thing and definitely not the best one but many of my planes go into that direction. I like them.
Nevermind... I just rebuilt it exactly like in the screenshot... It was just KSP doing weird stuff.
